1. Field of the Invention
The present invention relates generally to the field of radiation detection and imaging technology.
2. Description of Related Art
When a radiation particle (gamma ray, neutron, electron, etc.) is detected in a scintillation detector, the scintillation detector will emit light, which is then converted into an electronic signal by a photosensor (e.g., photomultiplier tube or photodiode). This electronic signal can then be received and processed by electronic circuits. In the period after radiation hits the scintillation detector, the scintillation light decays exponentially with a time constant xcfx84 (the time when the light level decays to 37% of the onset level), as shown in FIG. 1.
FIG. 1 shows energy output by two gamma ray particles over time. Since the total amount of light emitted by the scintillation detector represents linearly the energy deposited by the radiation particle in the detector, the area or integral under the curves in FIG. 1 is a measure of the particle energy. As shown in FIG. 1, area 5 and area 10 define a measure of the particle energy of the gamma ray particles. Furthermore the initial peak in the light level is also proportional to the radiation energy. Hence both the area 5 and peak V1 in FIG. 1 may be used to measure the energy of the gamma ray or radiation particle. Since the area under the curve (integral of light) includes many more light signals than the instantaneous peak light level, the integral (the total amount of light emitted) is generally used to measure the radiation energy.
As the radiation flux increases, it becomes increasingly likely that the next radiation particle may arrive at the detector while all previous events are still emitting light (FIG. 2). In this case, the identity of each individual radiation particle will be lost, and several particles will merge into one large signal, as shown in FIG. 2. In this case neither the peak level (V1 or V2 of FIG. 1), nor the integral information (area 5 or area 10 of FIG. 1) can be used to separate or measure the energy of each particle. In these situations, the detection system will fail to respond properly because of erroneous measurement.
It is known that it takes a time period of approximately 4xcfx84 to collect 98% of the scintillation light from each radiation excitation. Thus, if the next event arrives at time t greater than 4xcfx84, the pile-up-energy error on the next event will be less than 2%. Hence, to keep pile-up error small, it is desirable to minimize the chance that two events (radiation excitations) will occur in a time period less than 4xcfx84. Since the time-lapse between two events is a random distribution (i.e., the time-lapse between two events is a random variable) centered about the xe2x80x9caverage arrival timexe2x80x9d, it is generally practiced in the prior art to operate the detector so that the xe2x80x9caverage arrival timexe2x80x9d is 10xc3x97(4xcfx84)=40xcfx84, to lower the random chance of having two events coming closer than 4xcfx84. With this 10xc3x97 xe2x80x9chead-roomxe2x80x9d, the probability that two events will come closer than 4xcfx84 would be approximately 10% (using Poisson statistics). The head-room factor as a function of pile-up percentage is shown in Table 1 below:
Thus, a 10xc3x97 head-room is a reasonable choice, and is generally practiced in the prior art. When coupled with a 4xcfx84 light-collection time (system dead-time), such a prior art detector provides a measured-energy error (due to pile-up) of less than approximately 2% for approximately 90% of the time, and an energy measurement error (energy resolution) greater than 2% for approximately 10% of the time. This minimum 10xc3x97 head-room (40xcfx84) timing requirement means that the maximum detection-rate should be less than 1/(40xcfx84) for the scintillation detector.
The present invention permits a scintillation detector system to operate at a much higher event-rate (count-rate) by obviating the 10xc3x97 head-room factor without pile-up. The present invention maintains a greater event-rate with little sacrifice in the total amount of scintillation light collected, specifically at a 10 times higher radiation flux with little or no sacrifice in measurement accuracy. If the fraction of scintillation light collected can be reduced (i.e., if a user is willing to compromise measurement accuracy), the present invention allows the detector to count at count-rates approximately twenty times greater than conventional methods.
The present invention includes an apparatus for signal pile-up prevention, comprising a delay circuit for receiving, holding, and passing an incoming signal; a computation circuit for determining a weighted value of the incoming signal; a sampling circuit for receiving the weighted value. The sampling circuit passes the weighted value (which may be passed to an A/D converter) upon receipt of a triggering signal, which corresponds to receipt of a next incoming signal at a trigger circuit. In an exemplary embodiment, the computation circuit may comprise an amplifier, an integator, and an adder. In an exemplary embodiment, the weighted value is a sum of an integrated value and an instantaneous value, and may be a substantially constant value.
An apparatus according to the present invention may also include a smoothing circuit connected to the circuit adapted to receive the incoming signal. The apparatus may also comprise a residual subtraction circuit for reducing the weighted value by a residual signal value. The sampling circuit discharges said weighted value upon input of the triggering signal.
The present invention may be used in connection with nuclear medicine applications, such as a PET or gamma camera, and may be used to determine both energy and position information. Such an apparatus comprises a plurality of delay circuits, a plurality of computation circuits, and a plurality of sampling circuits, wherein each of the delay circuits receives a different incoming signal from a different output of a gamma camera. The delay circuit, computation circuit, and sampling circuit comprise a pile-up prevention circuit.
Particular embodiments will comprise a plurality of pile-up prevention circuits, and may include a digital signal processor and fast trigger connected to each of the pile-up prevention circuits. Such an embodiment may also comprise an inter-zone detection circuit connected to the fast trigger and a multi-zone-trigger processor connected to said inter-zone detection circuit, capable of centroid averaging. An exemplary embodiment may have a plurality of fast triggers.
A method for preventing signal pile-up according to the present invention may comprise: delaying an incoming signal for a preselected time prior to passing the incoming signal; computing a weighted value of the incoming signal; and sampling the weighted value upon receipt of a triggering signal from the next radiation particle, thereby preventing signal pile-up. Computing may include amplifying the incoming signal to obtain an amplified signal, integrating the incoming signal to obtain an integrated signal, and adding the amplified signal and integrated signal to obtain the weighted value. This method thereby creates a variable signal collection time.
Another method according to the present invention may determine position and energy information of incoming signals without pile-up. Such a method may include: delaying at least one prenormalized position signal and a total energy signal; computing a weighted value for each prenormalized position signal and the total energy signal; sampling the weighted value for each prenormalized position signal and the total energy signal, upon receipt of a triggering signal from the next radiation particle. In an exemplary embodiment, the prenormalized signals and the total energy signal may be corrected by subtracting remnant values of all previous signals.
In yet another aspect, the present invention comprises an apparatus for dynamically detecting energy of each one of a plurality of incoming signals received from a detector, without pile-up of previous incoming signals, including: a delay circuit connected to receive an incoming signal from the detector and to pass the incoming signal from an input to an output of the delay circuit after a time delay; a trigger circuit connected to receive the incoming signal from said detector, and for generating a triggering signal upon receipt of a subsequent incoming signals at the trigger circuit; a computation circuit connected to the output of said delay circuit for determining a weighted value of the incoming signal; a sampling circuit connected to receive the weighted value from said computation circuit, and for passing the weighted value from an input to an output of the sampling circuit upon receipt of the triggering signal; and a residual subtraction circuit connected to the output of said sampling circuit, for subtracting a residual signal value corresponding to a residual weighted value of previous incoming signals, and for providing an output signal corresponding to the energy of the incoming signal.
Another aspect of the present invention comprises an apparatus connected to a gamma camera for detecting position and energy information of each one of a plurality of incoming signals received by the gamma camera, without pile-up of previous incoming signals, including: a first delay circuit connected to receive a first incoming signal from the gamma camera, and for passing the first incoming signal from an input to an output of the first delay circuit after a first time delay; second and third delay circuit arranged like the first delay circuit to receive, delay, and pass, second and third signals; a trigger circuit connected to receive the third incoming signal from said gamma camera, and for generating a triggering signal and a timing mark upon receipt of a next third incoming signal at the trigger circuit; first, second and third computation circuits, each connected to receive an output of a respective one of the first, second, and third delay circuits, and for determining a respective weighted value for each of the first, second, and third incoming signals; first, second and third sampling circuits, each connected to receive a respective one of the first, second, and third weighted values, and for circuits passing the respective weighted value upon receipt of the triggering signal; and a digital signal processor connected to receive the first, second, and third weighted values, and for subtracting residual signal values corresponding to residual weighted values of previous ones of the incoming signals, and for providing an output signal corresponding to a position value of the first and second incoming signals and an energy value of the third incoming signal.
Another aspect of the present invention resides in a method of obtaining energy information for each one of a plurality of incoming signals received from a detector, without signal pile-up, comprising the steps of: delaying an incoming signal for a preselected time; computing a weighted value of the signal after the preselected time; sampling the weighted value upon receipt of a subsequent signal; and subtracting a residual signal value from the weighted value to obtain the energy information. The residual signal value may correspond to a residual weighted value of at least one previous incoming signal, thereby preventing signal pile-up.
Yet another aspect of the present invention resides in a method of determining position and energy information of a plurality of incoming signals from a detector without pile-up, comprising the steps of: receiving a first and second prenormalized position signal and a total energy signal from the detector; delaying the first and second prenormalized position signals and total energy signal for a preselected time, computing a weighted value for each of the first and second prenormalized position signals and total energy signal after the preselected time; and sampling the weighted value for each of the first and second prenormalized position signals and total energy signal upon receipt of a subsequent first and second prenormalized position signals and total energy signal.
The methods of the present invention may be used to operate gamma-cameras (or other radiation detectors) in very high count-rate situations. The present invention includes the following features: (a) no compromise in measured energy-resolution in low count rates; (b) count recoveries and accurate energy measurement even for gamma-rays within a pile-up involving multiple gamma-rays; (c) optimal scintillation-light collection in very high count-rate situations; and (d) ability to merge with a multi-zone architecture to further increase count-rate capability. The present invention includes algorithms that apply to all triggering gamma-rays (it is to be understood that although gamma-rays are discussed herein, the present invention applies to all types of radiation detectors), for extracting the correct energy and position of every triggering gamma-ray.